docs: def get_upstream_version(package):
      """Determine upstream version based on installed package
  
      @returns None (if not installed) or the upstream version
      """
      import apt_pkg
      cache = apt_cache()
      try:
          pkg = cache[package]
      except Exception:
          # the package is unknown to the current apt cache.
          return None
  
      if not pkg.current_ver:
          # package is known, but no version is currently installed.
          return None
  
      return apt_pkg.upstream_version(pkg.current_ver.ver_str)